Tractor and pots - electronics - POBC

[AdamQ]

I'm attempting to motorize a previously pedal-powered tractor for my son. I have the motor and a suitable motor controller - suitable in all respects except that the potentiometer (5KOhm) is 270° min to max and, since it's planned to be directly connected to the throttle pedal, something of the order 45° would be preferable. Before I get involved in mechanical solutions, I couldn't help wondering if there was a more elegant electrical solution and I knew that the POBC would help if it could ... (my search for a 5KOhm quarter-turn pot has proved fruitless).

 

I'm not too worried about throttle 'sensitivity' or anything - the pedal is likely to be at one extreme or the other most of the time.

 

Anyway, any suggestions would be gratefully received ...

 

Cheers,

Adam

[Bricol]

Stick a 270 degree quadrant pulley on it, and link to pedal with a cable - much like the throttle cable on you seven.

 

Bri

[Jonathan Kay]

My first thoughts were also mechanical, but involved Lego Technic or Meccano. Bri's solution already beats that.

 

If you want an electronic solution: What resistance does your controller want to see for no power, and what resistance for full power? I'm thinking of either:

* A potentiometer that gives the required resistance for full power at 1/6 of it's travel, or

* Some sort of passive voltage divider. (Wheatstone bridges for fuel senders discussed in this thread.)

 

Jonathan

 

Edited by - Jonathan Kay on 18 Dec 2012 21:18:41

[Grebby]

Is it possible to use a 20k 270deg POT and limit the travel of the it so you only get up to 5k (i.e. 67deg ish)?

[Richard Price]

Adam,

how many terminals are connected to your potentiometer?

 

Pot.'s are most commonly used as a potential divider.

 

If you had a 12V system, then you would connect 12V to one end, and 0v to the other. The wiper would then produce a voltage between 0V at one extreme of travel, and 12V at the other.

 

If it is used as a potential divider, providing a reference voltage as above, then, without some quite complex electrickery, I'm feeling that a mechanical solution is likely to be significantly easier.

 

If it IS being used as a resistor, then using just part of the range of a single pot is possible, but you'd really need to know and understand quite a few details to help you specify what network of resistors you'd need to construct around you pot to get the desired effect.

 

 

 

 

Edited by - Richard Price on 19 Dec 2012 15:18:10

[AdamQ]

Thanks for all the replies and suggestions. There are three terminals on the pot - there are details and a wiring diagram under the Downloads tab here.

 

In haste - will be back shortly!

 

Thanks again,

 

Adam

[MartinWoodham]

assuming it's going to be a low speed tractor anyway (?), you might be better off with a switch, possibly with two 'gears' ie two power settings.

 

Pots are generally only available as 270deg, sliders, or trimmer/ preset styles (which is what's on the controller baord now - and not designed for frequent use), so you'll need to rig up a mechanism.

 

Easiest suggestion - find the smallest/ cheapest throttle body you can off fleabay, mount it next to the controller board somewhere, and rig a cable to the pedal. Then connect the throttle pot to your controller board, replacing the preset style pot that's on the board.

 

Overly bulky I know, but someone has done the hard work for you in fixing the pot/ cable interface. You could always take the angry-grinder to the bits of throttle body you don't need....

[MartinWoodham]

BTW from the spec sheet on your link, the pot is a potential divider (it's the duty-cycle one you want) - so the value of it is immaterial. Within reason

[AdamQ]

Thanks for the further input - all most helpful.

 

Yes, it is going to be a low speed tractor - 5.2 mph is the theoretical maximum speed (assuming I did my sums right - if I missed out a 60 in going from RP Minute to MP Hour it's either going to be very fast or very slow!).

 

I like the TB idea - following the initial suggestion about a quadrant, I was thinking down the lines of cannibalising the old plastic 48 mm K-series TB I have somewhere, but it makes even better sense to make fuller use of it, and, like you say, there's always the 'angry-grinder' ...

 

Onwards and upwards ...

 

Thanks again all,

 

Adam

[virden]

I built an electic powered jeep for my lad and learnt the hard way that his friends have no mechanical sympathy. Don't forget to fit a circuit breaker between the throttle and the motor, it will stop it being burnt out, as they sit, foot hard on the throttle, with the car firmly parked against an imovable object

[revilla]

Looking at the wiring diagram, it looks like the throttle potentiometer is straight across the regulated supply providing essentially a variable voltage between 0v and the supply voltage to signal the motor speed required. Given that, a simple circuit with an operational amplifier chip and a couple of resistors (one of which could be variable to provide range adjustment) could be connected between the pot and the controller circuit board, without actually having to modify anything, which would map the voltage "input" from the pot over the required angular range of operation onto the appropriate "output" range, so the controller would see a signal as though the full 270 degrees of travel was in use. Certainly worth a try anyway, would only cost a few quid to put it together.